Followings have been required conventionally for mold steels, particularly used for plastic molding:
(1) good mirror polishability, and pinholes or other minute pits is less likely generated;
(2) high texturability;
(3) high strength, wear resistance and toughness;
(4) high machinability;
(5) high corrosion resistance and rust resistance;
(6) high thermal conductivity;
and the like.
Among them, the rust resistance and the thermal conductivity have been important properties to be improved for recent mold steels. That is, there is a problem that rust occurs on a mold surface due to dew condensation during non-use of the mold such as between production processes or during maintenance. If the rust generates on the mold surface, a step of removing the rust, such as polishing, becomes indispensable for using the mold, which causes reduction of productivity. Therefore, the rust resistance is strongly required to be improved for mold steels. In addition, high thermal conductivity of mold steels is an important property particularly for plastic molding which is subjected to repeat of heating and cooling, in order to shorten the thermal cycle to improve the productivity.
The applicant has proposed a mold steel excellent in the rust resistance and the thermal conductivity, the mold steel having a composition including, by mass % (hereinafter, represented merely as “%”): 0.07 to 0.15% of C; more than 0% and less than 0.8% of Si; more than 0% and less than 1.5% of Mn; less than 0.05% of P; less than 0.06% of S; more than 0% and less than 0.9% of Ni; 2.9 to 4.9% of Cr; one or both of Mo and W wherein an amount of (Mo+½W) is in a range of more than 0% and less than 0.8%; more than 0% and less than 0.15% of V; 0.25 to 1.8% of Cu, and the balance of Fe and inevitable impurities, and the mold steel having a hardness of 30 to 42 HRC (Patent Literature 1).
There has been also proposed a mold steel for plastic molding, having a composition including: 0.03 to 0.25% of C; 0.01% to 0.40% of Si; 0.10% to 1.50% of Mn; 0.30% or less of P; 0.050% or less of S; 0.05% to 0.20% of Cu; 0.05% to 1.50% of Ni; 5.0% to 10.0% of Cr; 0.10% to 2.00% of Mo; 0.01% to 0.10% of V; 0.10% or less of N; 0.01% or less of 0; and 0.05% or less of Al, and the balance of Fe and inevitable impurities and satisfying the following expressions: (Cr+Mo)≤10% and 7≤(Cr+3.3Mo) (Patent Literature 2).